In industry, there are a number of reasons for removing a surface coating from the base of a manufactured item, such as a defect in the coating, or to change the color of the coating, or to recover scrap. Also, the industrial painting process for such articles often mounts the bases to be painted on a hook carried by an overhead conveyor, and paint is applied to the base by immersion, as a liquid spray or a powdered coating. In such manufacturing processes, the hangers become covered with paint and require periodic stripping to prevent paint chips from the hangers falling on and damaging the newly applied coating during heat curing of the base.
There are three basic methods in the prior art for removing the surface coating from a base, namely, abrasive buffing, application of chemicals for removing the material of the coating, and pyrolysis. Abrasive buffing is a labor intensive process that contaminates the environment, requires replacement of abrasive materials, and requires skill to avoid damaging the base. Chemical removing methods require the use of strong and costly solvents, tends to be time consuming and results in a residue that generally poses a costly disposal problem.
Pyrolysis has been defined by the Encarte Dictionary as “the process of chemically decomposing solid wastes by heat in an oxygen-reduced atmosphere. This results in a gas stream containing primarily hydrogen, methane, carbon monoxide, carbon dioxide, and various other gases and inert ash, depending on the organic characteristics of the material being pyrolysized.” Pyrolysis is a relatively fast and inexpensive way to remove a surface coating, but prior art ovens tend to be hard to control and likely to damage the base.
The use of heat to thermally decompose a surface coating of paint is described in the patent art at least as early as 1922 (U.S. Pat. No. 1,416,865 granted on May 23, 1922 discloses an oven for removing enamel from fenders and the like). However, the thermal decomposition of the coating produces gases which are flammable, and burning of these gases produces heat in addition to the heat applied to the article to achieve pyrolysis. The liberation of additional heat increases the temperature within the oven tending to damage the base and creating control and smoke problems. In addition, the presence of gases from decomposition of the surface coating may produce an explosive mixture of gas and oxygen, thus increasing the likelihood of damage to the base and requiring precautions in the construction and operation of the oven.
One attempt to make an oven for burning-off the surface coating from the base is described in U.S. Pat. No. 5,351,632 granted on Oct. 4, 1984 to C. Mann in which the atmosphere within the oven is continually changed to prevent the build-up of vapors and smoke. Another approach is disclosed in U.S. Pat. No. 5,018,458 granted on May 28, 1991 to McIntyre et al. in which water is sprayed on the contents of the oven to maintain the temperature below a maximum value. Another approach has been to replace the atmosphere within the oven with an inert gas, or a vacuum (U.S. Pat. No. 4,141,373 granted on Feb. 27, 1979 to Kartanson et al.). In all of these prior art systems for removing a surface coating by pyrolysis, thermal energy is being given up for control, and in most cases equipment is added to the oven for the sole purpose of control.
While removal of coatings from a base is an important use of the present invention, the pyrolyic methods and ovens of the present invention are affective to disassociate organic materials for other purposes, such as disposal of waste materials commonly referred to as sludge, or the recovery of ingredients contained within the organic materials.